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Chromosomal Basis of Inheritance

Chapter 15

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Morgan & Drosophila Thomas Hunt Morgan - first to associate a specific gene

with specific chromosome in early 20th century Drosophila breeding

good genetic subject prolific 2 week generations 4 pairs of chromosomes

3 pair autosomes, 1 pair sex chromosome

XX=female, XY=male

1910 | 1933

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autosomal

chromosome

s

sexchromosome

s

Classes of chromosomes

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Wild Type

Morgan looked for varied traits Discovered a single male fly with white

eyes instead of the usual red The normal character phenotype is wild

type Alt traits

are mutant phenotypes

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Huh!Sex matters?!

F2generation

100%red-eye female

50% red-eye male50% white eye male

Discovery of sex linkage

P X

F1generation(hybrids)

100%red eye

offspring

true-breeding white-eye male

true-breedingred-eye female

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RR rr

What’s up with Morgan’s flies?

x

r r

R

R

Rr

Rr Rr

Rr

100% red eyes

Rr Rr

x

R r

R

r

RR

Rr rr

Rr

3 red : 1 white

Doesn’t workthat way!

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In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes

develop as a female: XX gene redundancy,

like autosomal chromosomes an X & Y chromosome

develop as a male: XY no redundancy

Genetics of Sex

X Y

X

X

XX

XY

XY

50% female : 50% male

XX

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XRXR XrY

Let’s reconsider Morgan’s flies…

x

Xr Y

XR

100% red eyes

XR

XRXr XRY

XRYXRXr

x

XRXr XRY

XR Y

XR

Xr

XRXr

XRYXRXR

XrY100% red females50% red males; 50% white males

BINGO!

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Sex-linked Gene Sex-linked genes appear on X chrom

Females (XX) may have 2 red-eyed alleles or may be heterozygous (all have red eyes)

Males (XY) have only 1 allele and will be red eyed ONLY if they have red-eyed allele (otherwise white allele – white)

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Genes on sex chromosomes Y chromosome

few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones

many effects = pleiotropy!

X chromosome other genes/traits beyond sex determination

mutations: hemophilia Duchenne muscular dystrophy color-blindness

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Sex-linked usually

means“X-linked”

more than 60 diseases traced to genes on X chromosome

Duchenne muscular dystrophyBecker muscular dystrophy

Ichthyosis, X-linkedPlacental steroid sulfatase deficiencyKallmann syndromeChondrodysplasia punctata, X-linked recessive

HypophosphatemiaAicardi syndromeHypomagnesemia, X-linkedOcular albinismRetinoschisis

Adrenal hypoplasiaGlycerol kinase deficiency

Incontinentia pigmentiWiskott-Aldrich syndromeMenkes syndrome

Charcot-Marie-Tooth neuropathyChoroideremiaCleft palate, X-linkedSpastic paraplegia, X-linked, uncomplicatedDeafness with stapes fixation

PRPS-related gout

Lowe syndrome

Lesch-Nyhan syndromeHPRT-related gout

Hunter syndromeHemophilia BHemophilia AG6PD deficiency: favismDrug-sensitive anemiaChronic hemolytic anemiaManic-depressive illness, X-linkedColorblindness, (several forms)Dyskeratosis congenitaTKCR syndromeAdrenoleukodystrophyAdrenomyeloneuropathyEmery-Dreifuss muscular dystrophyDiabetes insipidus, renalMyotubular myopathy, X-linked

Androgen insensitivity

Chronic granulomatous diseaseRetinitis pigmentosa-3

Norrie diseaseRetinitis pigmentosa-2

Sideroblastic anemiaAarskog-Scott syndrome

PGK deficiency hemolytic anemia

Anhidrotic ectodermal dysplasia

AgammaglobulinemiaKennedy disease

Pelizaeus-Merzbacher diseaseAlport syndrome

Fabry disease

Albinism-deafness syndrome

Fragile-X syndrome

Immunodeficiency, X-linked,with hyper IgM

Lymphoproliferative syndrome

Ornithine transcarbamylase deficiency

Human X chromosome

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Map of Human Y chromosome?< 30 genes on Y chromosome Sex-determining Region Y (SRY)

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Why is crossing over not as common with the sex chromosomes?

What types of genes are on the sex chromosomes?

If there is a gene at the top of the X chromosome, how many alleles does a man have for that gene?

Sex- linked gene

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Hemophilia

Hh x HHXHYXHXh

XHXh

XH

Xh

XHY

Y

XH

sex-linked recessive

XH Ymale / sperm

XH

Xh

fem

ale

/ eg

gs XHXH

XHXh

XHY

XhY

XHXH XHY

XHXh XhY

carrier disease

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X-inactivation Female mammals inherit 2 X chromosomes

one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random

patchwork trait = “mosaic”

XH

Xh

XHXh

patches of black

patches of orange

tricolor catscan only befemale

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Male pattern baldness Sex influenced trait

autosomal trait influenced by sex hormones age effect as well = onset after 30 years old

dominant in males & recessive in females B_ = bald in males; bb = bald in females

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1. If a gene for male pattern baldness is carried on the X chromosome, who in the family would pass the trait to their son?

2. Since females have two X chromosomes and males only have one, women end up with excess genetic info. What does the cell do to make up for this extra amount of DNA?

3. So, if one X chromosome is turned off, why are women not as susceptible as men to sex-linked traits such as color-blindness, balding, MD, and hemophilia?

Learning Check

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Cell 1- Diploid cell w/ homologous chromosomes. traits A and B are on different chromosomes. Heterozygous. Chance that they will produce gamete with genotype AB?

aB? Ab? Or ab? (50%) Cell 2- genes A and B are on opposite ends of the same

chromosome Chance that they will produce gamete with genotype AB?

aB? Ab? Or ab? (recombination frequency will be 50% due to crossing over)

Cell 3- genes are on the same chromosome close together Probability of recombination decreases because the genes

are less likely to cross over, segregate and independently assort. They will be “linked”.

Gene linkage

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As genes are located closer to one another, what happens to their recombination frequency?

Learning Check

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If gene A recombines 12% of the time with gene B, but 16% of the time with gene C, which gene is closer to gene A?

If gene B and C recombine with one another 28% of the time, what is the order of genes A, B, and C along the chromosome?

Recombination frequencies

B A C

1612

28

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The following is a map of four genes on a chromosome:

Between which two genes would you expect the highest frequency of recombination? A) A and W B) W and E C) E and G D) A and E E) A and G

 

Learning Check

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Lab 7: Genetics (Fly Lab)

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Lab 7: Genetics (Fly Lab) Description

given fly of unknown genotype use crosses to determine mode of inheritance of trait

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Lab 7: Genetics (Fly Lab) Concepts

phenotype vs. genotype dominant vs. recessive P, F1, F2 generations sex-linked monohybrid cross dihybrid cross test cross chi square

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Lab 7: Genetics (Fly Lab) Conclusions: What is the genotype of the

parent generation?

Case 1

Case 2